Background: People with severe to profound intellectual disabilities have particular needs for others to help them regulate stress. Such support may be hampered by difficulties in detecting and interpreting expressions of stress. This study aimed to describe and identify stress-related behaviours and sensitive, stress-regulating responses to these behaviours. Method: Video recordings of psychotherapeutic interactions between four clients with severe intellectual disabilities and behavioural problems and two attachment therapists were analysed using a stepwise spiral of analysis approach. Stress-related behaviours were identified with a behaviour rating scale for arousal and valence. Results: Distinct stress-related behaviours were subdivided into stress-specific, non-stress-specific, and client-specific behaviours. Additionally, examples of responsive behaviours to clients' expressions of varying arousal and valence were found. Conclusions: A wide variety of expressions indicative of different levels of arousal were found, with few expressions specific for stress. The descriptions of responsive behaviours might support parents and caregivers in external stress regulation.